Substrate-triggered bipolar junction transistor and ESD protection circuit

ABSTRACT

An ESD protection circuit using a novel substrate-triggered lateral bipolar junction transistor (STLBJT) for providing a discharging path between power rails. The ESD protection circuit comprises an ESD detection circuit and a STLBJT device. The STLBJT device formed in a P-type substrate includes N-type collector and emitter regions coupled to the power rails, respectively. The substrate region between the collector and emitter regions, on which there is no field oxide device, serves as a base of the STLBJT device. The STLBJT device further includes a first P-type region coupled to the ESD detection circuit and a second P-type region coupled to one of the power rails, which are spatially separated from the collector/emitter regions, respectively. The STLBJT device is turned on by substrate-triggering responsive to the signal coming from the ESD detection circuit and establishes the discharging path between the power rails.

This application is a Divisional of co-pending Application Ser. No.10/309,225, filed on Dec. 4, 2002, for which priority is claimed under35 USC § 120; the entire contents of which are hereby incorporated byreference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an electrostatic discharge (ESD)protection technology, more specifically, to a substrate-triggeredlateral bipolar junction transistor (STLBJT) for use in ESD protectionand the related ESD protection circuit.

2. Description of the Prior Art

ESD phenomena have become a reliability issue in complementary metaloxide semiconductor (CMOS) integrated circuits (ICs) because oftechnology scaling and high frequency requirements. For radio frequency(RF) ICs, the on-chip ESD protection design suffers from severallimitations, such as low parasitic capacitance, constant inputcapacitance, insensitivity to substrate coupling noises, and high ESDrobustness. A typical requirement of the maximal loading capacitance foran RF input pad is only 200 fF at an operating frequency of 2 GHz. Asdescribed herein, this 200 fF target not only includes ESD protectiondevices but also the bond pad itself. In order to fulfill theserequirements, diodes are commonly used for ESD protection in I/Ocircuits. To deal with these challenges, low-capacitance bond pad andlow-capacitance ESD protection circuitry had been proposed with somespecific techniques.

Moreover, by adding a turn-on efficient ESD clamp circuit across thepower rails of the input ESD protection circuit formed by the diodes,the overall ESD level of the input pin can be significantly improved.FIG. 1 is a circuit diagram of a conventional input ESD protectioncircuit with a power-rail (VDD-to-VSS) ESD clamp circuit. In FIG. 1, ESDdiodes Dp1 and Dn1 are connected to pad 10 and ESD diodes Dp2 and Dn2are connected to pad 12. Numeral 14 represents an internal circuit andnumeral 16 represents the ESD clamp circuit connected between the VDDand VSS power rails. When the ESD pulse is applied to pad 10 and pad 12is relatively grounded, the ESD current is conducted to the power railVDD through the forward-biased ESD diode Dp1. The ESD current on the VDDpower rail is discharged to the VSS power rail by the efficientVDD-to-VSS ESD clamp circuit 16. Finally, the ESD current is conductedto grounded pad 2 through the forward-biased ESD diode Dn2. The overalldischarging path of the ESD current is indicated by a bold line 18 inFIG. 1. By using such ESD protection design, the ESD diodes are alloperating in the forward-biased condition to discharge the ESD current.The diode operated in the forward-biased condition can sustain a muchhigher ESD level with a small device dimension. Thus, the ESD clampdevice in the input ESD protection circuit can be realized with smallerdevice dimensions to significantly reduce the input capacitance of theinput ESD protection circuit for high-frequency applications.

Therefore, the turn-on efficient power-rail ESD clamp circuit cansignificantly improve the ESD robustness of IC products if thepower-rail ESD clamp circuit can be turned on efficiently while an ESDevent is happening.

In addition, U.S. Pat. No. 5,744,842 disclosed an area-efficientVDD-to-VSS ESD protection circuit. FIG. 2 is a circuit diagram of thisESD protection circuit and FIG. 3 is a cross-section of the ESDprotection circuit.

As shown in FIG. 2 and FIG. 3, the ESD protection circuit is composed ofan ESD transient detection circuit 102 and an N-type field oxide device100. The ESD transient detection circuit 102 comprises aresistance-capacitance network, which includes a resistor R and acapacitor C and is connected between the VDD and VSS power rails, and aninverter 104 including a PMOS transistor Mp and an NMOS transistor Mn.The field oxide device 100 is a parasitic bipolar junction deviceincluding an N+ collector 302, N+ emitter 304 and a P+ base 300. The RCnetwork has a delay constant longer than the duration of theelectrostatic pulse and shorter than the duration of the rising time ofVDD power-on. The contact of the resistor R and the capacitor C iselectrically coupled to the input of inverter 104. The gate of PMOStransistor Mp is coupled to the gate of NMOS transistor Mn. The sourcesof PMOS transistor Mp and NMOS transistor Mn are coupled to the VDD andVSS power lines, respectively. The drains of PMOS transistor Mp and NMOStransistor Mn are coupled to base 300 of the field oxide device 100.When the ESD pulse occurs on the VDD power line and the VSS power lineis relatively grounded, the gates of PMOS transistor Mp and NMOStransistor Mn maintain a low voltage level since the RC network has alonger delay time constant. Thus, PMOS transistor Mp turns on and NMOStransistor Mn turns off. The initial ESD current flows into the base 300of the field oxide device 100 through PMOS transistor Mp and then flowsto the VSS power line through the substrate pickup 308. At the sametime, the initial ESD current raises the base voltage of the parasiticBJT and triggers on the parasitic BJT. Then, the ESD current on the VDDpower line flows to the VSS power line through the parasitic BJT.

In '842, the base of the parasitic BJT is defined by a field oxide.However, field oxide can be replaced by shallow trench isolation (STI)in sub-quarter-micron CMOS process because of bird's beak effect of thefield oxide. The STI is deeper than the field oxide in siliconsubstrate. Therefore, the field oxide device in STI technology is hardto turn on.

U.S. Pat. No. 5,581,104 disclosed a grounded-base BJT device serving asan ESD protection device. This grounded-base BJT structure includes aparasitic diode used to aid the triggering of the BJT.

In the above, the power-rail ESD clamp circuit is important to improvethe ESD robustness of IC products. As well, the power-rail ESD clampcircuit needs to be triggered efficiently while an ESD event ishappening. However, the field oxide device in sub-quarter-micron CMOSprocess using the STI technology is hard to turn on.

Therefore, the object of the present invention is to provide an ESDprotection device and an ESD protection circuit using the same, whichhas a lower triggering voltage and can be triggered more efficiently asthe ESD event occurs, especially in the sub-quarter-micron CMOS process.

SUMMARY OF THE INVENTION

The present invention achieves the above-indicated object by providingan ESD protection circuit for providing a low-resistance path fordischarging the ESD current between a first line and a second line, suchas between the VDD and VSS power rails and between other signal lines.The ESD protection circuit comprises an ESD detection circuit and alateral bipolar junction transistor (BJT). The function of the ESDdetection circuit is to trigger the lateral BJT as an ESD event occurs,and to send a grounding signal or other signals to turn off the lateralBJT as the circuit is operated normally. The lateral BJT is coupled toan output terminal of the ESD detection circuit and used to provide adischarging path between the first line and the second line. In thepreferred embodiments, the lateral BJT is formed in a P-type substrateand includes N+ collector and emitter regions coupled to the first lineand the second line, respectively. The substrate region between thecollector and emitter regions, on which there is no field oxide or STI,serves as a base of the lateral BJT. In addition, in the P-typesubstrate, there are first/second P-type doped regions spatiallyseparated from the collector/emitter regions, respectively. The firstP-type doped region is connected to the output terminal of the ESDdetection circuit and the second P-type doped region is connected to thesecond line. In addition, STI structures are preferably formed betweenthe collector region and the first doped region and between the emitterregion and the second doped region, where the STI structure between theemitter region and the second doped region can be omitted. In theabove-mentioned ESD protection circuit, the lateral BJT can be easilytriggered by substrate-triggering through the first doped regionresponsive to the trigger signal coming from the ESD detection circuit,forming the discharging path between the first line and the second line.

Moreover, the substrate-triggered lateral BJT can further comprise somedoping regions to enhance the BJT performance. For example, an N-wellcan be added between the base and the substrate pickup to increase thebase resistance. For another example, an N-type region can be added toincrease the area of the collector of the lateral BJT.

Further scope of the applicability of the present invention will becomeapparent from the detailed description given hereinafter. However, itshould be understood that the detailed description and specificexamples, while indicating preferred embodiments of the invention, aregiven by way of illustration only, since various changes andmodifications within the spirit and scope of the invention will becomeapparent to those skilled in the art from this detailed description.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will become more fully understood from thedetailed description given hereinbelow and the accompanying drawings,which are given by way of illustration only, and thus are not limitativeof the present invention, and in which:

FIG. 1 is a circuit diagram of a conventional input ESD protectioncircuit with a VDD-to-VSS ESD clamp circuit;

FIG. 2 is a circuit diagram of a conventional ESD protection circuit;

FIG. 3 is a cross-section of the ESD protection circuit shown in FIG. 2;

FIG. 4 is a cross-section of the STLBJT device in accordance with thefirst embodiment of the present invention;

FIG. 5 is a block diagram of the ESD protection circuit using the STLBJTdevice as an ESD protection device in accordance with the firstembodiment of the present invention;

FIG. 6 is a circuit diagram of the ESD protection circuit shown in FIG.5;

FIG. 7 is a partial cross-section of the ESD protection circuit inaccordance with the first embodiment of the present invention;

FIGS. 8 a-8 d are cross-sections illustrating the manufacturing processof the STLBJT device in the present embodiment.

FIG. 9 is a layout view of the STLBJT device in accordance with thefirst embodiment of the present invention;

FIG. 10 is a cross-section of the STLBJT device in accordance with thesecond embodiment of the present invention;

FIG. 11 is a cross-section of a first example of the STLBJT device inaccordance with the third embodiment of the present invention;

FIG. 12 is a cross-section of a second example of the STLBJT device inaccordance with the third embodiment of the present invention;

FIG. 13 is a cross-section of a third example of the STLBJT device inaccordance with the third embodiment of the present invention;

FIG. 14 is a cross-section of a fourth example of the STLBJT device inaccordance with the third embodiment of the present invention;

FIGS. 15-17 are cross-sections of the STLBJT devices containing P-typedoped regions with different profiles; and

FIG. 18 is a cross-section of the STLBJT device in accordance with thefifth embodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION First Embodiment

The main concept of the present invention is to utilize a novelsubstrate-triggered lateral bipolar junction transistor (STLBJT) deviceas an ESD protection device. FIG. 4 is a cross-section of the STLBJTdevice in accordance with the first embodiment of the present invention.As shown in FIG. 4, the STLBJT device comprises two N-type regions 22and 24 and two P-type regions 44 and 45 inside a P-type substrate 20.Both the N-type regions 22 and 24 are located between the P-type regions44 and 45. The N-type regions 22 and 24 are close but not connected, andisolated by a substrate region 20A of P-type substrate 20. The N-typeregion 22, the P-type substrate region 20A and the N-type region 24constitute the collector, base and emitter of the STLBJT device Q1.There is no STI or field oxide formed on the base of the STLBJT deviceQ1 (region 20A) between the N-type regions 22 and 24. Therefore, theSTLBJT device can be triggered more efficiently than the conventionalfield oxide device. In addition, there are STI structures 33 and 32between the N-type region 22 and the P-type region 45 and between theN-type region 24 and the P-type region 44, respectively, where thesymbol R_(E) denotes the substrate resistance. In operation, the P-typeregion 45 receives a signal to bias the base of the STLBJT device Q1 andtriggers on the STLBJT device Q1 to bypass the ESD current.

FIG. 5 is a block diagram of the ESD protection circuit including theSTLBJT device Q1 in accordance with the first embodiment of the presentinvention. The ESD protection circuit provides a discharge path betweenpad 1 and pad 2, such as the pads of the VDD and VSS power rails, andincludes a STLBJT device Q1 and ESD detection circuit 77. The collectorand emitter of the STLBJT device Q1 are respectively coupled to pad 1and pad 2 and its gate is coupled to the output of the ESD detectioncircuit 77. The ESD detection circuit 77 is designed to detect the ESDevent across the pad 1 and the pad 2. When an ESD event occurs, theSTLBJT device Q1 is substrate-triggered to provide a path fordischarging the ESD current. As described above, the advantage of thepresent embodiment is that the STLBJT device Q1 can be triggered moreefficiently than the conventional field oxide device, thus the ESDprotection performance is strengthened.

FIG. 6 is a circuit diagram of an example of the ESD protection circuitshown in FIG. 5. As shown in FIG. 6, the ESD detection circuit 77 iscomposed of a resistor R, a capacitor C, a PMOS transistor Mp and anNMOS transistor Mn. The RC network including resistor R and capacitor Chas a delay constant longer than the duration of ESD pulses and shorterthan the initial rising time of the signal on pad 1. As ESD pulses areapplied to pad 1 and pad 2 is relatively grounded, node A maintains thelow voltage level due to the longer delay constant of the RC network.Thus, the PMOS transistor Mp is turned on and the NMOS transistor Mn isturned off. Subsequently, the ESD current flows into the base of theSTLBJT device Q1 through the PMOS transistor Mp, triggering on theSTLBJT device Q1. Thus, the ESD current flows from pad 1 to pad 2through the turned-on STLBJT device Q1.

FIG. 7 is a partial cross-section of the ESD protection circuit inaccordance with the first embodiment of the present invention, where theSTLBJT device is illustrated with a cross-section and the ESD detectioncircuit 77 with a circuit diagram. As shown in FIG. 7, the output of theESD detection circuit 77 is coupled to P-type region 45. In addition,the N-type regions 22 (i.e. the collectors) of the STLBJT devices Q11and Q12 are coupled to pad 1, the N-type regions 24 (i.e. the emitters)are coupled to pad 2 and the P-type regions 44 are also coupled to pad2. The signal coming from the ESD detection circuit 77 can trigger theSTLBJT devices Q11 and Q12, respectively, to provide paths fordischarging the ESD current.

FIGS. 8 a-8 d are cross-sections illustrating the manufacturing processof the STLBJT device in the present embodiment. At first, in FIG. 8 a, aP-type substrate 20 is prepared. In FIG. 8 b, an active region mask 95is defined by a photolithographic process and formed on the substrate20. Then, the P-type substrate 20 is etched and filled with insulatorslike silicon dioxide or silicon nitride to form STI structures 32 and33. In FIG. 8 c, an N-type implantation mask 96 is defined by aphotolithographic process. The N-type regions 22 and 24 are formed withan implantation of N-type impurities 88 using the N-type implantationmask 96. Finally, in FIG. 8 d, a P-type implantation mask 97 is definedby a photolithographic process. Then the P-type regions 44 and 45 areformed with an implantation of P-type impurities 89 using the P-typeimplantation mask 97. The photolithographic process defining the P-typeimplantation mask 97 shown in FIG. 8 d can be performed earlier thanthat defining the N-type implantation mask 96 shown in FIG. 8 c. Usingthe process shown in FIGS. 8 a-8 d, the STLBJT device like that shown inFIG. 4 is formed on the P-type substrate 20.

FIG. 9 is a layout view of the STLBJT device in accordance with thefirst embodiment of the present invention. As shown in FIG. 9, there arethree key photo masks used to form the STLBJT device, including an oxidedefinition (usually called OD) photo mask for defining implantationregions, a P-type implantation photo mask for defining the base of theSTLBJT device and an N-type implantation photo mask for defining theN-type collector and emitter of the STLBJT device. The cross-section inrespect of the line VII-VII′ is illustrated in FIG. 7. It is noticedthat the above-mentioned photolithographic processes are available inand process-compatible with the current sub-quarter-micron CMOS process.Thus, there is no need to change or add new process steps to theexisting process, which is favorable to the industrial application.

Second Embodiment

The structure of the STLBJT device disclosed in the first embodiment ofthe present invention is not intended to limit the scope of theinvention. In respect of different requirements, this device structurecan be modified to achieve the same or better ESD protectionperformance. FIG. 10 is a cross-section of the STLBJT device inaccordance with the second embodiment of the present invention. Thedifference in the STLBJT structures shown in FIG. 10 and FIG. 7 (thefirst embodiment) is that the STI structure 32 between N-type region 24and the P-type region 44 is omitted, which can still achieve the objectof the present invention.

Third Embodiment

In the present invention, an extra N-type doped region can be added inthe STLBJT device structure shown in the first embodiment to increasesubstrate resistance R_(B) or change emitter concentration, expeditingthe triggering operation. FIG. 11 is a cross-section of a first exampleof the STLBJT device in the present embodiment. As shown in FIG. 11, anN-type doped region 211 is formed under the N-type region 24 (i.e. theemitter) and on the side of the N-type region 24, that is, surroundingthe N-type region 24, to increase the substrate resistance R_(B). FIG.12 is a cross-section of a second example of the STLBJT device in thepresent embodiment. As shown in FIG. 12, an N-type doped region 212 isformed just under the N-type region 24 to increase the emitterconcentration. The formation of the N-type doped regions 211 and 212 ispreferably made by the n-well formation step or the ESD implantationstep.

In addition, the above-mentioned manipulation can also be applied to thecollector of the STLBJT device. FIG. 13 is a cross-section of a thirdexample of the STLBJT device in the present embodiment. As shown in FIG.13, an N-type doped region 251 is formed under the N-type region 22(i.e. the collector) and on the side of the N-type region 22, that is,surrounding the N-type region 22. FIG. 14 is a cross-section of a fourthexample of the STLBJT device in the present embodiment. As shown in FIG.14, an N-type doped region 252 is formed just under the N-type region 22to adjust the substrate resistance and the collector concentration.

Fourth Embodiment

In the present invention, an extra P-type doped region can be added inthe structure of the STLBJT device shown in the first embodiment toincrease base resistance or base impurities. FIG. 15, FIG. 16 and FIG.17 are cross-sections of the STLBJT devices containing P-type dopedregions with different profiles. In FIG. 15, a P-type doped region 261is formed under the substrate region 20A and the N-type regions 22 and24. In FIG. 16, a P-type doped region 262 is formed under the substrateregion 20A and laterally adjacent to the N-type regions 22 and 24. InFIG. 17, a P-type doped region 263 is formed under the substrate region20A and its width is narrower than that of the substrate region 20A. Theextra P-type doped regions shown in FIG. 15, FIG. 16 and FIG. 17 areused to increase the base resistance or the base impurity concentration.

Fifth Embodiment

In the above-mentioned third and fourth embodiments, extra N-type orP-type doped regions are formed in the substrate to improve thetriggering performance of the STLBJT device in the present invention. Inthe present embodiment, several doped regions are formed in differentregions to optimize the performance of the STLBJT device. FIG. 18 is across-section of the STLBJT device in the present embodiment. As shownin FIG. 18, an N-type doped region 256 formed by ESD implantation isadded under the N-type region 24 (i.e. the emitter), an N-type dopedregion 255 formed by the N-well process is added under the N-type region22 (i.e. the collector), and a P-type doped region 268 formed by theP-well process is added under the P-type region 45. In this case, theimpurity concentration of N-type ESD implantation region 256 is higherthan that of N-well region 255, which is further higher than that ofP-well region 268. Thus, the triggering performance of the STLBJT devicecan be optimized.

According to the above description, the advantages of the STLBJT deviceserving as an ESD protection device and the ESD protection circuit usingthe same are as follows:

-   -   1. In the sub-quarter-micron process, the STLBJT device of the        present invention can be triggered more efficiently than the        field oxide device adopted in the conventional scheme.        Therefore, the ESD protection circuit using the STLBJT device of        the present invention as an ESD protection device can provide        higher resistance to the ESD current.2. The manufacturing        process of the STLBJT device of the present invention is fully        process-compatible with current sub-quarter-micron CMOS        processes and does not require adding extra photo mask        processes, and is thus easily incorporated to the current        production line.

While the invention has been described by way of example and in terms ofthe preferred embodiment, it is to be understood that the invention isnot limited to the disclosed embodiments. On the contrary, it isintended to cover various modifications and similar arrangements aswould be apparent to those skilled in the art. Therefore, the scope ofthe appended claims should be accorded the broadest interpretation so asto encompass all such modifications and similar arrangements.

1. A method of forming a lateral bipolar junction transistor on asubstrate of a first type, comprising the steps of: forming a firstshallow trench isolation structure on the substrate of the first typeusing an oxide definition photolithographic process; forming a firstregion and a second region of a second type in the substrate using asecond-type implantation photolithographic process, the first region andthe second region being spatially separated by a substrate regionconstituted by the substrate; and forming a third region and a fourthregion of the first type in the substrate and spatially separated fromthe first region and the second region, respectively, in a first-typeimplantation photolithographic process, the first shallow trenchisolation structure being formed between the first region and the thirdregion.
 2. The method of claim 1, wherein the method further comprises astep of forming a second shallow trench isolation structure locatedbetween the second region and the fourth region in the oxide definitionphotolithographic process.
 3. The method of claim 1, wherein the oxidedefinition photolithographic process, the first-type implantationphotolithographic process and the second-type implantationphotolithographic process are process-compatible to the existing CMOSprocess.
 4. The method of claim 1, wherein the first type is the N typeand the second type is the P type.
 5. The method of claim 1, wherein thefirst type is the P type and the second type is the N type.